High-bandwidth technologies which are prevalent nowadays use the existing copper wire infrastructure deployed for plain old telephone systems (POTS) communication. With the advent of the Internet and other high-bandwidth electronic communication networks and systems and consumer demand increasing for information, such as interactive gaming and electronic entertainment like video on-demand, there has been a substantial need for reliable and affordable high bandwidth media for facilitating data transmissions between service providers and their customers. In relation to this need existing copper wire telephone systems infrastructure is utilized for delivering broadband services.
One such broadband technology is Digital Subscriber Line (DSL), which comes in multiple variations such as ADSL, HDSL, IDSL, SDSL, RADSL and VDSL (collectively “xDSL”). Asymmetric digital subscriber line (ADSL) provides a higher data rate downstream (i.e., to the customer) than upstream (i.e., to the service provider).
For service providers to effectively provision new customers in a less-costly manner, information about existing lines must be acquired prior to provisioning service. One critical task prerequisite to provisioning of xDSL services is loop length determination, which must be done to qualify the loop for DSL service. Loop link determination also qualifies the types of service that may be provisioned for a specific customer. In addition, the presence of load coils (inserted at specific intervals along the loop to enhance and restore call quality), bridge taps, (un-terminated cables on a communications line within the local loops, for tapping into a line) and other line characteristics affect the loop's ability to support DSL and must be considered when provisioning DSL service. Other equipment installed on subscriber loops may also render the loop unsuitable for the provision of DSL service.
In DSL applications, it is useful to be able to determine the achievable rate given actual line conditions of any given loop. This facilitates the provisioning and maintaining of services, such as DSL services. One thing that makes this task difficult is that measurements of the loop characteristics must be taken with access to only one end of the loop, typically the Central Office (CO). Such testing and provisioning is referred to as single end loop testing or single end line testing (SELT). Achievable data rate can be computed from the loop attenuation and the noise present. Noise can be measured directly. Loop attenuation can be computed if the loop length and composition are known.
In practical applications, it is desirable to combine a DSL modem with the SELT measurement device. They share the same transmit and receiver circuitry, as well as the digital processing unit. While the benefit in cost and ease of use is obvious, such arrangement brings a major complication in the design of SELT measurement method, as explained below.
In a SELT measurement, the received signal contains two components: the far-end echo (FE) and the near-end echo (NE). FE contains information on the loop termination at the far end, and the loop length. NE is mainly determined by the loop interfacing circuitry of the measuring device, and is not necessary when determining loop length in SELT measurement. However, because of the loop attenuation, FE signal is much weaker than NE. Therefore, it is necessary to isolate contribution from the FE signal, in order to conduct accurate SELT measurements. In a DSL modem, FE and NE components overlap in space and time. The ability to separate the individual FE and NE components and artifacts and residue is a major challenge in successful SELT measurements.
Testing methods and apparatuses that enable the determination of the physical and electrical characteristics of subscriber loops to be determined is all well known. Such methods and apparatuses are taught, for example, in U.S. Pat. No. 4,105,995 to Bothof et al.; U.S. Pat. No. 4,870,675 to Fuller et al.; and, U.S. Pat. No. 5,881,130 to Zhang. Single-ended qualification of customers for an xDSL service is disclosed by U.S. Pat. No. 6,266,395 to Liu et al. Traditionally, service providers in public switched telephone networks (PSTN) dispatch a technician to a customer premises who has expressed an interest in xDSL service. The technician coordinates testing with another technician at the service provider's Central Office (CO). This labor intensive process increases operating overhead and delays service provisioning. For comprehensive testing, full broadband test access is required.
Single End Loop Testing (SELT) may be used to extract information about the transmission environment, or loop, in a DSL system from reflective measurements. Extracted information may include characteristics or effects introduced by the transmission medium (e.g., the wireline), the noise environment in which the transmission medium operates, characteristics or effects introduced by DSL devices on the transmission medium, etc. The information about the transformation environment may include, for example, loop length, loop termination type, and the like. For instance, SELT may involve injecting signals into a loop under test at a central office (CO) to determine the loop capability to support different kinds of DSL services. Typically, only the CO is involved in loop testing and SELT is used to extract reflected signal information (often referred to as far-end echo) from the measurements. Near-end Echo, even at the tail of the near-end echo, is much stronger than the far-end echo causing the latter to be completely obscured in the correlated background noise. Much effort has been made to tackle the SELT problem and standardization by the ITU under project G.selt is currently underway. So far ITU contributions have focused on the calibration process, which treats the signal path between the transmitter, the receiver, and the loop as a three port linear time invariant (LTI) network. The process computes the input impedance via a one port scattering parameter.
Therefore, a need exists for effectively provisioning DSL services using single-end line testing that enables robust and accurate measurement of loop length and loop termination types.